1. Field of the Invention
The present invention relates to an organic field effect transistor and a method for producing such a transistor.
2. Description of Related Art
Currently known organic field effect transistors comprise:                drain and source electrodes,        a semiconductor layer made of an organic semiconductor material arranged between the drain and source electrodes,        at least one gate electrode, and        an electrically insulating layer interposed between the gate electrode and the semiconductor layer with the insulating layer being directly in contact with the semiconductor layer,        
When a potential is applied to the gate electrode, the charge carriers present in the semiconductor layer concentrate at the interface between the semiconductor layer and the insulating material, remaining confined to the semiconductor layer. This concentration of charge carriers then forms the conduction channel which is characteristic of the “on” state of the transistor.
Organic field effect transistors are produced using organic semiconductor materials. Such organic field effect transistors are also referred to by the abbreviation OFET.
An organic semiconductor or an organic semiconductor material is an organic compound in the than of a crystal or polymer which exhibits properties that are similar to those of inorganic semiconductors. These properties include conduction by electrons and holes and the presence of an energy gap. These materials gave rise to organic electronics.
The mobility μ of the charge carriers in an organic semiconductor is defined by the following equation in the absence of a magnetic field and in the steady-state regime:{right arrow over (V)}=μ{right arrow over (E)}  (0)where:                {right arrow over (V)} is the velocity of the charge carriers in the organic semiconductor, and        {right arrow over (E)} is the permanent electrostatic field.        
Mobility μ is expressed in centimeters squared per volt per second (cm2V−1 s−1).
The mobility of charge carriers in organic semiconductors currently remains well below that in inorganic semiconductors; it does not exceed 20-35 cm2·V−1·s−1 whereas, in inorganic semiconductors, it is of the order of 103 cm2·V−1·s−1. There is a proportionality relationship between the mobility of the charge carriers and the electrical conductivity a of a material which can be expressed as follows:σ=pqμ  (1)where:                q is the charge of the charge carriers,        p represents the volume concentration of the charge carriers, and        μ represents the mobility of the charge carriers.        
The Ion/Ioff ratio is one of the criteria for measuring the quality of a transistor. This ratio Ion/Ioff is the ratio of the intensity of current Ion which flows through the transistor when the latter is in the “on” state to the intensity of the current Ioff which flows through the same transistor under the same conditions when it is in the off or “blocked” state. In particular, current intensity Ion and current intensity Ioff are measured with the same voltage Vds between the drain and the source.
In known organic transistors, the semiconductor layer is made by using a single identical organic semiconductor whose mobility, at micrometer scale, is homogeneous throughout the semiconductor.
In order to increase the maximum current intensity Ion which can flow through the transistor for several tens of seconds without damaging it, attempts are currently being made to use organic semiconductors which have the highest possible mobility. There are, for example, organic semiconductors which have a mobility in excess of 10−1 cm2V−1 s−1 or even 1 cm2V−1 s−1; these can be used to produce the semiconductor layer.
However, for a transistor with a given geometry, increasing the maximum current intensity Ion does not necessarily result in an increase in the Ion/Ioff ratio because of the proportionality relationship between charge carrier mobility and conductivity. In fact, for most organic semiconductors, the higher the mobility, the more the conductivity of the material increases and current Ioff therefore increases. Here, the geometry of a transistor is deemed to remain constant if the distance L between the opposite-facing faces of the drain and source electrodes and the length W of the channel which separates the drain and source electrodes remain constant. These parameters L and W are described in greater detail later on in this description.
The best organic transistors currently have an Ion/Ioff ratio which peaks at around 105.